Every node in a communication network that desires to send data must have a transmitter. For instance, in a cellular communication network, transmitters are found in base stations as well as user equipment devices, or terminals. FIG. 1 illustrates one embodiment of a conventional transmitter 10. The transmitter 10 may be a transmitter of a wireless base station or a transmitter of a user equipment device. As shown, the transmitter 10 includes a digital-to-analog converter (DAC) 12 followed by a modulator and upconverter 14. The output of the modulator and upconverter 14 is amplified by a power amplifier (PA) 16 and then filtered by a filter 18. FIG. 2 illustrates another embodiment of the conventional transmitter 10, which is similar to that illustrated in FIG. 1. However, in this embodiment, the transmitter 10 includes a digital predistorter (DPD) 20 that operates to predistort the input signal to compensate for a non-linearity of the PA 16. An adaptor 22 adaptively configures the digital predistorter 20 based on the input signal and a feedback signal provided from the output of the PA 16 via a demodulator and downconverter 24 and an analog-to-digital converter (ADC) 26.
Ideally, the filter 18 is a linear filter. However, all analog filters, including the filter 18, have a non-linear characteristic region. The non-linear characteristic of the filter 18 results in several disadvantages when the transmitter 10 is implemented in a base station of a cellular communication network for 4-th Generation (4G) cellular communication networks such as, for example, Long Term Evolution (LTE) cellular communication networks. More specifically, next generation cellular communication networks will have macro base stations that operate at a high output power (e.g., ˜80 watt (W) average power and ˜400 W peak power) and low power base stations that operate at a lower output power (e.g., 0.1 to 10 or 20 W). With respect to the high power base stations, the only type of filter that can handle the high output power is a cavity filter. However, due to the non-linearity issue of the cavity filters the first round production yield of the cavity filter manufacturing is just around 60%. As such, manufacturing of cavity filters is very expensive. Therefore, there is a need for improving the non-linearity issue of the cavity filters to reduce the manufacturing cost.
As for the low power base stations, cavity filters can easily handle the desired output power while providing the desired linearity. However, cavity filters are very large and heavy as compared to other types of filters (e.g., ceramic filters or monoblock filters). Therefore, the use of cavity filters is not desired for the low power base stations. However, the ceramic filters and monoblock filters do not provide the desired linearity when operating in higher power, for example, higher than 5 W. Thus, there is a need for improving the non-linearities of the ceramic filters and monoblock filters so that the low power base stations operating in power range from 5 W to 20 W can use such filters instead of using the bulky cavity filter.
The conventional transmitter 10 also results in issues when implemented in user equipment devices for 4G cellular communication networks such as, for example, LTE cellular communication networks. Specifically, when the conventional transmitter 10 is implemented in a user equipment device, the filter 18 is a miniature filter such as a Surface Acoustic Wave (SAW) filter, a Film Bulk Acoustic Resonator (FBAR) filter, or a Bulk Acoustic Wave (BAW) filter. The peak power of the PA 16 in 4G cellular communication networks could push up the filter 18 to work in its non-linear region. As a result, the non-linearity of the filter would be an issue to the system. So, there is also a need for improving the nonlinearities of the miniature filters so that such miniature filters can be used in the user equipment devices for 4G wireless networks without communication quality degradation.
All of the issues discussed above stem from non-linearity that the filter 18 will show when it works above a certain power level and in its non-linear characteristic region. Therefore, there is a need for systems and methods for decreasing or eliminating the non-linearity of a non-linear filter.